User Manual

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2009 Industrial Wireless Guidebook
Cellular Networks
3
The SCADA system reads the measured
flow and level, and sends the setpoints to the PLCs
PLC1 compares the measured flow to
the setpoint, controls the speed pump as
required to match flow to setpoint
PLC2 compares the measured level to
the setpoint, controls the flow through
the valve to match level to setpoint
‘‘Mix-and-Match” SCADA
SCADA/HMI systems evolved in three stages, including Monolithic, Distributed, and Network SCADA
systems. Monolithic SCADA involves an independent system for single station and uses a vendor’s
proprietary communication protocols. As the number of monitoring sites increased, multiple stations were
required for monitoring and control in a Distributed SCADA system. The introduction of LAN technology
in the late 1990s provided SCADA systems with real-time monitoring capabilities. At the time, most
communication protocols were proprietary.
Due to a limited choice of equipment when requirements changed, open communication protocols became
increasingly popular, such as Modbus RTU and Modbus ASCII (originally both developed by Modicon) over
RS-485. By 2000, most I/O makers offered completely open interfacing such as Modbus TCP over Ethernet
and IP.
Today, Network SCADA systems, which use open system architecture, standards, and protocols, distribute
functionality across a WAN rather than a LAN. It is now easier to connect third party peripheral devices
because of the adoption of information technology. IT field protocols, such as Internet Protocol (IP), are
used for communication between the master station and communications equipment. Due to the use of
standard protocols, many Network SCADA systems are accessible from the Internet.
SCADA systems are coming in line with standard networking technologies. Ethernet and TCP/IP based
protocols are replacing the older proprietary standards. A key protocol is OPC Client/Server protocol.